* add create_file() helper function
* create files using "x" mode instead of "w" to detect when a previous test
  forget to remove a file
* open file for writing in unbuferred mode (buffering=0)
* replace "try/finally: unlink" with self.addCleanup(support.unlink)
* register unlink cleanup function *before* creating new files

* Use context manager on urllib objects to ensure that they are closed on error
* Use self.addCleanup() to cleanup resources even if a test is interrupted
  with CTRL+c

* Replace "try/finally: os.remove()" with self.addCleanup(support.unlink) or
  self.addCleanup(support.rmdir): the support function handles the case when
  the file doesn't exist
* Replace "try/finally: f.close()" with "with open(...) as f:"
* test_getsize: add a second test with a different size
* Create file using "x" mode to ensure that the file didn't exist before, to
  detect bugs in tests
* Open files in unbuffered mode (buferring=0) to write immediatly data on disk
* Replace map() with simpler code
* Split isdir() unit test into two units tests to make them less dependant,
  same change for isfile() test
* test_samefile(): test also two different files

Issue #25911:

* Try to fix test_os.BytesWalkTests on Windows
* Try to mimick better the reference os.DirEntry on Windows
* _DummyDirEntry now caches os.stat() result
* _DummyDirEntry constructor now tries to get os.stat()

* Update code for the name regrtest output format.
* Enhance also test_regrtest test on --fromfile

Issue #25911: Use support.check_warnings() to expect or ignore
DeprecationWarning in test_os.

Issue #26588: Optimize memory footprint of _tracemalloc before non-zero domain
is used. Start with compact key (Py_uintptr_t) and also switch to pointer_t key
when the first memory block with a non-zero domain is tracked.

On Windows, socket.setsockopt() raises an OverflowError if the socket option is
larger than INT_MAX bytes.

Issue #18787: restore "bin" name in test_spwd but catch KeyError.

Ignore empty lines in stderr.

Issue #18787: try to get the "root" entry which should exist on all UNIX
instead of "bin" which doesn't exist on OpenIndiana.

We know that Py_SIZE(b) is -1 or 1 an so fits into the sdigit type.

Don't check if the dict key exists before getting the key. Instead get the key
and handle error.

Restore the previous code for stack_overflow().

Issue #23848, #26622:

* faulthandler now only logs fatal Windows exceptions.
* write error code as decimal, not as hexadecimal
* replace "Windows exception" with "Windows fatal exception"

Only display duration if a test takes more than 30 seconds.

wcsnlen_s() result type is size_t.

Don't return a void value.

Issue #23848: On Windows, faulthandler.enable() now also installs an exception
handler to dump the traceback of all Python threads on any Windows exception,
not only on UNIX signals (SIGSEGV, SIGFPE, SIGABRT).